1. Field of the Invention
The invention is directed to an improved supply system for a medium, in particular for treating exhaust gases of an internal combustion engine, to an improved exhaust gas cleaning device, and to method for operating such a supply system.
2. Description of the Prior Art
To reduce the nitrogen oxides contained in the exhaust gas of an internal combustion engine, for such engines, especially those with self-ignition, a method for selective catalytic reduction has proved advantageous. In this method, the nitrogen oxides together with ammonia are converted into nitrogen and water in a selective catalytic converter. The medium required for the catalytic conversion of the nitrogen oxides, such as a reducing agent, is carried along in the vehicle, instead of the ammonia, in the form of an aqueous urea solution, from which the ammonia can be liberated by hydrolysis of the urea solution, in whatever quantity is needed for the conversion. When the fuel tank is being filled, the reducing agent is introduced into a separate storage tank and supplied to the exhaust gas flow via a supply line to a metering unit, such as a metering valve, disposed directly on the exhaust gas tube.
To assure perfect function of the metering valve, this valve must on the one hand be cooled. On the other hand, depending on the urea concentration, there is the risk that the aqueous urea solution will freeze at certain temperatures. The expansion behavior of the urea solution is similar to that of water. If the freezing urea solution cannot expand, then impermissibly high pressures can occur in the interior of components that are filled with the urea solution, and these pressures can cause the destruction of those components. It is known, in the work chamber of a metering valve, to provide walls or elements that at least partially yield to pressure and that can reversibly compensate for a pressure in the work chamber that occurs when ice forms. The alternative in which at least one region between a pump and the metering unit can be evacuated oppositely to the pumping direction in normal operation is also known. Damage to the metering unit from unwanted ice formation at low outdoor temperatures, and in particular a buildup of excessively high ice pressures in the assembly, however, is not reliably avoidable by those provisions.
To prevent the medium from heating up too severely, it must be assured that the amount of heat that is absorbed by the cooling medium at the metering unit is dissipated to the environment again. This is preferably done by the metering unit via a return line that leads back to the storage tank. A problem here is that the storage tank must be thermally insulated, to prevent freezing of the medium for as long as possible. Moreover, the lines must be insulated so that thawing of the medium located in the lines after it has frozen can be accomplished as quickly as possible and in a way that reduces high energy losses. Hence there is a conflict in terms of the aims between the best possible insulation with a view to the freezing behavior of the medium and the best possible insulating action, to assure heat dissipation.